Methane emission reduction by adding sulfate to liquid dairy manure

Dairy farmers are interested in reducing the carbon footprint of milk. Reducing methane (CH₄) emissions is a key part of this goal, and manure is a significant CH₄ source. Technologies like anaerobic digesters for biogas production are effective; however, adoption rates are slowed by upfront costs and infrastructure needs. Achieving near-term emission reductions needs low-cost alternatives that can be quickly and widely adopted. Previous studies have shown that "acidification" of manure by adding sulfuric acid (H₂SO₄) suppressed CH₄ emissions; however, widespread adoption may be hindered by the challenge of handling acid on farms. This laboratory study was performed for 157 days at 24°C, and compared the efficacy of a sulfate-based non-acidic fertilizer (CaSO₄), and two rates of acidification, one at pH > 7 and one at pH < 7, for a sulfate-based acid (H₂SO₄) and a sulfate-free acid (H₃PO₄). Methane suppression by CaSO₄ at multiple rates was also analyzed. Two mechanisms of suppression were observed: acidification had a demonstrable early effect, lowering cumulative CH₄ emission within 40 days by up to 65% for H₂SO₄ and 54% for H₃PO₄, while sulfate-containing compounds showed increasing suppression after 50 days. Final cumulative CH₄ suppression was up to 63% for CaSO₄ and 91% for H₂SO₄, while H₃PO₄ was least effective. These results suggest H₂SO₄ is highly effective due to the combination of acidity and sulfate. Adding sulfate alone (CaSO₄) was more effective than adding acid alone (H₃PO₄). Hence, sulfate-based additives-like gypsum-may hold promise as an alternative near-term solution for dairy farms to make large CH₄ reductions.